Battery capacity display for a power tool

ABSTRACT

The present invention discloses a battery capacity display circuit for a power tool. The battery capacity display circuit has a first circuit module arranged in the battery pack of the power tool for detecting and displaying the battery capacity and a second circuit module arranged in the power tool for displaying the battery capacity. The battery capacity display also has an optical signal transmitting device and an optical signal receiving device to communicate with the first circuit module and the second circuit module.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to CN 201010199754.6, filed Jun. 9, 2010, and CN 201010204598.8, filed Jun. 11, 2010, which are hereby incorporated by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

TECHNICAL FIELD

The present invention relates to a battery capacity display circuit for a power tool.

BACKGROUND OF THE INVENTION

Power tools powered by battery packs usually are provided with the function of displaying the remaining battery capacity. The remaining battery capacity can be displayed by LED lamp, LCD screen, or the like. In general, the remaining capacity indicating device is on the battery pack. For instance, an LCD can be installed on the side of the battery pack to display the remaining battery capacity. An LCD can also be installed on the top of the body of the tool. But when the user is operating the tool the remaining capacity indicating device on the side of the battery pack or on the top of the body of the tool may not be visible to the user.

Other power tools have the remaining capacity indicating devices on both the battery packs and tools. The signals between the two remaining capacity indicating devices are transferred through using telecommunication technology which requires a transmitting device and receiving device installed on both the tool and the battery pack. The size of the transmitting device and receiving device may be large and with negatively affect the size of the DC power tool. Also, the transmission signal is not always stable and may be easily interfered with by the electromagnetic signals in the environment and may be limited to only a short distance. As a result, the displayed remaining capacity may not be consistent across the displays.

SUMMARY OF THE INVENTION

To overcome the disadvantage and deficiency of the prior art, the present invention provides a battery capacity display assembly which improves these problems. To address the above described issues, the battery capacity display circuit assembly disclosed in the present application, which can be used in power tools, may include a first circuit module, arranged in the battery pack of the power tool for detecting and displaying the battery capacity; a second circuit module, arranged in the power tool for displaying the battery capacity. The battery capacity display assembly may also have an optical signal transmitting device and an optical signal receiving device which communicate with the first circuit module and the second circuit module.

The photo-electric signal transmitting device and receiving device could be an IR transmitting circuit and receiving circuit, or a fiber optic apparatus. Also, since the battery capacity indicating devices are installed on both battery pack and the tool, the operator can observe the remaining capacity indicating devices from all angles conveniently during operation of the tools and the remaining capacity of the battery pack could be known before the battery has run out. Further, using a photo-electric signal provides some advantages such as the indicated results of both indicating devices are almost identical, and the signals are stable and accurate. Finally, the transmission is not easily interfered with and results in a lower cost.

BRIEF DESCRIPTION OF THE DRAWINGS

To understand the present invention, it will now be described by way of example, with reference to the accompanying drawings in which:

To understand the present application, it will now be described by way of example, with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of a power tool according to the present application;

FIG. 2 is a block diagram of a battery capacity displaying circuit of a power tool according to the present application;

FIG. 3 is a principle diagram of a photoelectric signal transmitter and receiver of a power tool according the present application;

FIG. 4 is a structural diagram of a fiber-optic apparatus of a power tool according to the present application;

FIG. 5 is a block diagram of a control circuit of a power tool according to the present application;

FIG. 6 is a circuit diagram of a control circuit of a power tool according to the present application;

FIG. 7 is a flowchart of a control circuit of a power tool according to the present application; and,

FIG. 8 is a circuit diagram of a battery capacity calculating circuit according to the present application.

DETAILED DESCRIPTION

The invention will now be explained with reference to the drawings and examples below. In ordinary operating situations, the DC power tool comprises a switch, a battery pack containing multiple rechargeable batteries, and a motor installed in the housing of the tool. Battery capacity displaying devices, for displaying the status of the battery pack capacity, are installed on the body of the tool and the battery pack. Using electrical drills as an example, as shown in FIG. 1, both the body 16 of the electrical drill and the battery pack 15 have battery capacity indicators. A body battery capacity indicator 14 is installed on the top of the body 16 of the tool, and a battery pack capacity indicator 13 is installed at the rear side of the battery pack 15. The battery pack capacity indicator 13 and the body battery capacity indicator 14 are preferably bicolor LED lamps in red and green which are capable of displaying three colors. The lamps display red when the red LED lamp is on, green when the green LED lamp is on, and orange when both of the red LED lamp and the green LED lamp are on. In the present example, the green lamp indicates that the remaining battery capacity of the battery pack is in “full” state, orange lamp indicates that it is in “medium” state, and red lamp indicates that it is in “low” state. The LED indicating lamps could also be replaced by an LCD display.

FIG. 2 shows a circuit diagram of the battery capacity displaying circuit of the DC power tool. The displaying circuit comprises a first circuit module 1 and a second circuit module 2, wherein the first circuit module 1 is positioned on the battery pack and comprises a battery detection circuit 3, a calculating control circuit 4, a battery capacity indicating circuit 9, and an optical signal transmitting device 6. The second circuit module 2 is positioned on the body of the tool, and comprises an optical signal receiving device 7 and a battery capacity indicating circuit 8.

When the battery capacity multi-position displaying circuit is in operation, in the first circuit module 1 on the battery pack, the battery detection circuit 3 will detect each of the voltage of the battery, the temperature of the battery and the discharge current of the battery. The battery detection circuit 3 then feeds the detection results to the calculating control circuit 4. The calculating control circuit 4 calculates and judges the detection results, and feeds the calculation results to the battery capacity indicating circuit 9 which includes a capacity indicator, so as to display the remaining battery capacity. The specific method of detecting and calculating will be elucidated below. The capacity signal is sent to the optical signal receiving device 7 on the body by the optical signal transmitting device 6. The optical signal receiving device 7 feeds the signal received to the battery capacity indicating circuit 8.

Preferably, the optical signal transmitting device 6 is a photoelectric signal transmitting circuit, and the optical signal receiving device 7 is a photoelectric element. The calculation results generated by the calculating control circuit 4 are provided to the optical signal transmitting device 6 in the form of a signal of a high-low electrical level. The optical signal transmitting device 6 sends the level signals to the second circuit module 2 through photoelectric transmitting means. The optical signal receiving device 7 in the second circuit module 2 is configured to receive the photoelectric signals transmitted by the first circuit module 1 on the battery pack, and to provide them to the battery capacity indicating circuit 8. The battery capacity indicating circuit 8 contains a battery capacity indicating device, which in the present example is a bicolor LED lamp, and controls the bicolor LED lamp to display different colors according to a signal of a different level. In an alternative example, the battery capacity indicating circuit 8 displays on an LCD displayer in the form of digit, histogram lamp, or others, so as to indicate the remaining capacity of the battery.

As shown in the principle diagram of the photoelectric signal transmitter and receiver in FIG. 3, the transmitter and receiver of the photoelectric signals comprises a photoelectric signal transmitting circuit 39 on the battery pack and a photoelectric signal receiver and displaying circuit 28. The photoelectric signal transmitting circuit 39 includes photoelectric elements 19, 20 for transferring the electrical signal to an optical signal and then transmitting the optical signal. The photoelectric signal receiving and displaying circuit 28 includes photoelectric element 40, for receiving and transferring the optical signal to an electrical signal for display by the battery capacity indicating circuit 8. The photoelectric element can be a photoconductive resistor, an infrared transmitting and receiving element, a PIN diode, etc. The current example uses an infrared transmitting element and infrared receiving element. The photoelectric signal transmitting circuit 39 used by the present application includes infrared transmitting tubes 19, 20 for the optical signal transmitting. The photoelectric signal receiving and displaying circuit 28 includes a photoelectric element 40 and battery capacity indicating circuit 8 which includes a power source 21 and a battery capacity indicator. The photoelectric element 40 includes infrared receiving tubes 25, 26. The battery capacity indicator includes battery capacity indicating circuit 8 and LED lamps 22, 23. The infrared receiving tubes 25, 26 receive either of the two branches of infrared signals, so as to respectively control the operation of LED lamp 22 and LED lamp 23.

When the photoelectric signal transmitting circuit 39 and the photoelectric signal receiving and displaying circuit 28 are connected, the control ends 17, 18 in the photoelectric signal transmitting circuit 39 receive two branches of level control signals from the calculating control circuit 4, and the photoelectric signal transmitting circuit 39 converts the level control signals to optical signals and transmits them. Once received by the photoelectric signal receiving and displaying circuit 28, the optical signals are converted to level signals and are used to cause the control and display device to display a corresponding result. When both of the two branches of control signals received by the photoelectric signal transmitting circuit 39 are at a high level and the circuits are on, the infrared signal transmitting tubes 19 and 20 transmit infrared signals, and the infrared signal receiving tubes 25 and 26 in the photoelectric signal receiving and displaying circuit 28 receive infrared signals. As a result, the LED lamp 22 and LED lamp 23 are also on. However, when both of the two branches of control signals are at a low level, the infrared signal transmitting tubes 19 and 20 do not transmit infrared signals, and the infrared signal receiving tubes 25 and 26 are off, and therefore the LED lamp 22 and LED lamp 23 are also off. When one of the two branches of control signals is at a high level and the other is at a low level, the LED lamp in the same loop of the infrared signal receiving tube receiving the high level signal is on, while the LED lamp in the same loop of the infrared signal receiving tube receiving the low level signal is off.

In another example, the optical signal transmitting and receiving structure can be realized by a fiber-optic apparatus. As shown in the structure diagram of a fiber-optic apparatus in FIG. 4, the fiber-optic apparatus may include a first optical guiding element 36 which is in the battery pack 15 and is used as an optical signal transmitting apparatus with a second optical guiding element 37 in the body 16 being used as optical signal receiving apparatus. The first optical guiding element 36 is formed of an optical guiding fiber 36 c and an optical guiding cylinder 36 b. There is a first optical guiding surface 36 a on the end of the optical guiding fiber 36 c close to the body 16, and on the other end of the optical guiding fiber 36 c connected to the optical guiding cylinder 36 b. The second optical guiding element 37 is formed of optical guiding fiber 37 c and optical guiding cylinder 37 b. There is also a first optical guiding surface 37 a on the end of the optical guiding fiber 37 c close to the battery pack 15 and on other end of the optical guiding fiber 37 c connected to the optical guiding cylinder 37 b. The optical guiding cylinder 36 b of the first optical guiding element 36 corresponds with the battery pack capacity indicator 13 for transmitting the optical signals from battery pack capacity indicating lamps. This is convenient for the conduction of the optical guiding fiber 36 c, 37 c. The optical guiding cylinder 37 b of the second optical guiding element 37 corresponds with the body capacity indicator 14 for transmitting the optical signals from optical guiding fiber 37 c and is convenient for the display of body capacity indicator 14.

After battery pack 15 has been installed on body 16, the first optical guiding surface 36 a of the first optical guiding element 36 and the second optical guiding surface 37 a of the second optical guiding element 37 are facing each other, and the optical signals can be transferred between the optical guiding fibers 36 c, 37 c. The optical signals from battery pack capacity indicator 13 are transferred to the body 16 through the optical guiding fiber 36 c, 37 c via the optical guiding cylinder 36 b. The optical signals are transferred using optical guiding cylinder 37 b of the second optical guiding element 37 and are then passed on to the body battery capacity indicator 14 to make the body battery capacity indicator 14 irradiate and make the light radiated by the body battery capacity indicator 14 in accordance with the optical signals radiated by the battery pack capacity indicator 13.

Since battery capacity indicating devices are installed on both the battery pack and the body of the tool, the operator can observe the remaining capacity indicating device from any convenient angle during operation of the tools and the remaining capacity can be known at any time. Also, the battery capacity indicating devices on the battery pack and the body of the tool display the output from the same battery detection circuit and calculating control circuit which results in the two signals being in sync with each other. Compared with traditional communication means, the photoelectric transmitting and receiving means and the fiber-optic transmission means are more resistant to interferences resulting in a more stable and accurate signal. As an added benefit, the above described signal transmitting and receiving means are provided with lower apparatus costs when compared with traditional communication transmitting and receiving structures.

FIG. 5 is a block diagram of the power tool control circuit. The circuit comprises a switch 10, a motor 11, a battery detection circuit 3, a calculating control circuit 4, a battery 12 and a battery capacity indicating circuit 9. The power tool control circuit is the circuit which is positioned in the battery pack, and detects, calculates and indicates the remaining capacity of the battery pack and further controls the work of the motor.

When the power tool control circuit is in operation, the operator presses the switch 10 of the power tool, and the battery detection circuit 3 starts detecting. The battery detection circuit 3 comprises the detection of the voltage, current and battery temperature of the battery 12, and feeds the detection results to the calculating control circuit 4 after the detecting is done. The calculating control circuit 4 comprises a semiconductor switch and a CPU for carrying out the calculating, judging and controlling. The CPU can also be replaced by micro controllers such as MCU or ARM, and the semiconductor switch can also be replaced by MOSFET. The CPU compares, calculates and judges the voltage, current and battery temperature detected by the battery detection circuit 3 respectively within a predefined voltage value, predefined current value and predefined battery temperature value. The CPU feeds a control signal after the calculating and the judging to the motor 11, the battery capacity indicating circuit 9 and the illumining and indicating circuit 5, so as to control whether the motor 11 operates or not, to control the state of display of the battery capacity indicating circuit 9 and to control the operation status of the illumining lamp. The specific procedures of comparing, calculating and judging will be elucidated below.

FIG. 6 is a specific circuit diagram of the power tool control circuit, and the circuit comprises a battery pack 33, a switch 10, a motor 11, a calculating control circuit 4, a battery capacity indicating circuit 9, an illumining and indicating circuit 5, a power circuit 32, and a battery detection circuit constituted by a voltage detection circuit 29, a current measure and calculate circuit 30 and a battery temperature detection circuit 31.

The power circuit 32 provides operational power for every circuit in the power tool control circuit except for the motor 11 and the illumining and indicating circuit 5. Wherein, a base voltage source U3 cooperates with the transistor Q6 to form a constant voltage. The resistors R32, R33 and R30 adjust the voltage, applied on the two ends of every circuit except for the motor 11 and the illumining and indicating circuit 5, to the required stable voltage value. Generally, the battery pack 33 provides unstable voltage varying from 8.1V to 12.4V, and the voltage is adjusted as a stable voltage of 3.3V after being decreased and regulated by the power circuit 32.

In the voltage detection circuit 29, after the switch 10 is closed, point a is in the high electrical level, and the transistor Q7B is saturated and on, thus the transistor Q7A is saturated and on. The voltage of the battery pack 33 is applied to point b, and is divided by R34 and C16. The pin 11 of the CPU measures the voltage at point c, thus the CPU calculates the voltage at point b, namely the voltage value of the battery pack 33. And the voltage detected by voltage detection circuit 29 is the voltage of battery 33, switch, and lead wires.

In the battery temperature detection circuit 31, a temperature sensor t_(NTC) is a thermal resistor and attached to the outer wall of the batter pack. The pin 11 of the CPU measures the voltage at point g after the voltage is divided by R47 and t_(NTC), so as to obtain the temperature of the battery. If the battery pack is over-temperature, the resistance of the temperature sensor t_(NTC) decreases. Thus, the CPU judges whether the battery pack is over-temperature according to the voltage at the point g.

In the current measure and calculate circuit 30, the pin 12 of the CPU measures the voltage at point h and obtains the voltage between two ends e and f of Q9 according to the voltage-dividing relationship of R50 and R51. Given the on resistance of Q9, the CPU calculates the current value I of the battery pack. When the CPU determines that the voltage value, the temperature value and the current value of the battery pack meet the start requirement of the motor 11, the pin 2 of the CPU outputs a low electrical level, the transistor Q8 is cut off, and the voltage at point d is applied on the pin 1 of the FET Q9, so that the Q9 is saturated and on, and motor 11 operates. In other embodiments, the current measure and calculate circuit 30 can be separated from the battery detection circuit.

In the illumining and indicating circuit 5, the control signal is provided by pin 9 of the CPU. When the control signal keeps at high level, transistor Q10 is on and the illumining and indicating lamp 34 is on when the control signal is at low level or doesn't exist, the transistor Q10 is cut off, and the illumining and indicating lamp 34 is off. When the control signal alternates between high level and low level, the transistor Q10 is on and cut off alternately, and the illumining and indicating lamp 34 flickers. When the illumining and indicating lamp 34 is a white LED lamp, it provides illumining source for the power tool in normal situations but flickers to provide alert signals when the power tool is in abnormal status.

FIG. 7 is a flowchart of the power tool control circuit. After the remaining battery capacity displaying circuit starts operating, the program first initializes the CPU responsible for the calculating and then carries out the judging for the battery voltage. If the battery voltage is below a predefined voltage value A1 of all the batteries, the CPU outputs a low level signal to the semiconductor switch. Thus, the semiconductor switch is off and the motor won't work. The CPU sends a judging signal to the battery capacity indicating circuit 9 at the same time, and the red LED lamp is on to indicate the under-voltage of the battery. Further, the CPU could divide the measured voltage value by the number of the battery units, and obtain the voltage of one single battery. If the voltage of one single battery is below a predefined voltage value A2 of one single battery, the red LED lamp is off; and if the voltage of one single battery is above the predefined voltage value A2 of one single battery, the red LED lamp keeps on, and the comparing and judging for the voltage value of one single battery may be repeated at the same time.

If the battery voltage is above or equal to the predefined voltage value A1 of all the batteries, the CPU outputs a high level signal to the semiconductor switch. Then the semiconductor switch is on, and the program proceeds to judge the discharge temperature of the battery.

If the battery temperature is above or equal to the predefined value C, the CPU outputs a low level signal to the semiconductor switch. Thus, the semiconductor switch is off and the motor won't operate. Meanwhile the CPU sends a judging signal to the illumining and indicating circuit 5, and the illumining and indicating lamp 34 keeps flickering to alert. At this time, the battery is in the state of over-temperature, and the alert from the illumining lamp could make the user notice this situation of over-temperature, so as to avoid the battery pack from causing danger due to over-temperature.

If the battery temperature is below the predefined value C, the CPU outputs a high level signal to the semiconductor switch. Thus, the semiconductor switch is on, the motor operates, and the program proceeds to judge the discharge current of the battery.

If the discharge current of the battery is above or equal to a predefined value B, the CPU outputs a low level signal to the semiconductor switch. Thus, the semiconductor switch is off and the motor stops operating. Meanwhile the CPU sends a judging signal to the illumining and indicating circuit 5, and the illumining and indicating lamp 34 keeps flickering to alert the user. When the discharge circuit is in the state of over-current, and problems such as motor damage would be caused if the motor kept operating in this situation, the alert from the illumining lamp could notify the user of over-current, so as to avoid the motor from being damaged. If the battery temperature is below the predefined value B, the CPU outputs a high level signal to the semiconductor switch. Thus, the semiconductor switch is on and the motor keeps operating and the program proceeds to calculate the battery capacity.

In the calculation of the battery capacity, voltage is used to calculate the capacity. Generally, the discharge loop of the battery pack includes several internal resistances like battery pack internal resistance, contact pin internal resistance, contact wine internal resistance, loop internal resistance, etc. In this invention, contact pin internal resistance, contact wine internal resistance, and the internal resistance of copper foil of PCB compositively relate to R_(c), and the battery pack internal resistance relates to R_(b). As the circuit diagram of the capacity calculating circuit shows in FIG. 8, it comprises a battery pack 33, a switch 10, a motor 11, semiconductor switch 35, and loop internal resistance R_(c) in which battery pack 33 includes battery 38 and battery pack internal resistance R_(b), semiconductor switch 35 includes MOSFET Q9 and semiconductor switch internal resistance R_(mos). As a result of the resistance having different resistance values at different temperatures, a temperature compensation coefficient C_(t) can be pre-established for measuring and calculating the battery pack internal resistance R_(b) exactly and counterbalance the deviation of the battery packs internal resistance value from the temperature. The temperature compensation factor C_(t) is the measure and coefficient of the battery pack internal resistance when the battery pack is working normally.

When the voltage detection circuit 29 works, the value of detected voltage V′ is a voltage between point m and point n, namely voltage between point m and the ground. The voltage value V_(mos) is voltage between two sides of semiconductor switch 35, namely voltage between point m′ and point n. The current value I of this loop is determined by the current measure and calculate circuit 30 by dividing voltage V_(mos) between point m′ and point n by the semiconductor switch internal resistance R_(mos).

In the capacity calculating circuit, the relationship of battery pack 33 voltage V_(b), voltage V_(c) between two sides of loop internal resistance R_(c) and detected voltage V′ is:

V _(b) =V _(c) +V′

When the power tool is operating, the capacity available to the motor 11 is the capacity of battery pack 33 minus the battery pack internal resistance R_(b), loop internal resistance R_(c), and semiconductor switch 35. Therefore, remaining battery capacity displayed by battery capacity displaying devices is decided by open circuit voltage V of battery 38, while the open circuit voltage V of battery 38 is the voltage of battery 33 minus the voltage V_(Rb) which is voltage between two sides of battery pack internal resistance R_(b). The voltage relationship in battery pack 33 is:

V _(b) =V′+C _(c) +V _(Rb)

Combining the relational expressions above, the calculating formula of voltage being displayed, namely, open circuit voltage V of battery 38 is:

V=V′+V _(c) +V _(Rb)

This results in the voltage being displayed is the summation of detected voltage V′, voltage V_(Rb) which is voltage between two sides of battery pack internal resistance R_(b), and voltage V_(c) between two sides of loop internal resistance R_(c). As a result of the same current in the capacity calculating circuit, the value of current is the current value I in the current measure and calculate circuit 30. Putting I into the calculating formula above, the voltage being displayed is:

V=V′+I·R _(c) +I·R _(b) ·C _(t)

This measuring method of the battery capacity considers not only the voltage consumed by battery pack internal resistance R_(b) and loop internal resistance R_(c), but also the deviation of the internal battery resistance brought on by the increase in the discharge temperature of the battery. Thus, the open circuit voltage V of battery 38 is more exact, and the display of the battery capacity can be used by motor 11, in fact, is more accurate.

The program then proceeds to display the battery capacity. In displaying the capacity, the status of the remaining battery capacity, obtained in the calculation of the capacity, is indicated to the user via battery capacity indicating device such as LED or LCD or buzzing device. Here, the battery capacity indicating device is a multi-colored LED lamp. In the present embodiment, it is a bicolor LED lamp in red and green, and it is capable of displaying red, orange and green. In case that the remaining battery capacity, obtained in the calculation of the capacity, is larger than or equal to a first predefined capacity value Q1, green is displayed; in the case that the remaining battery capacity is less than or equal to a second predefined capacity value Q2, red is displayed; and in the case where the remaining battery capacity is less than the first predefined capacity value Q1 but larger than the second predefined capacity value Q2, orange is displayed. The first predefined capacity value Q1 is larger than the second predefined capacity value Q2. Preferably, the first predefined capacity value Q1 is 50% of the gross battery capacity, and the second predefined capacity value Q1 is 10% of the gross battery capacity. In this way, the different colors of the lamp correspond to different remaining battery capacities, and the status of the remaining capacity can be clearly indicated to the user. Green stands for full capacity and the user could use the tool normally; red stands for insufficient capacity, and the tool cannot be used and needs recharging; orange stands for medium status, which is that the capacity is not full, but the tool can be used for a certain time. After that, the program returns to the judging for the battery voltage, and repeats this procedure.

Different colors of the capacity indicating lamp can clearly indicate the status of the remaining capacity of the battery to the user, and the user can know whether and how long the tool can operate.

In another example, the multi-colored LED lamp can be replaced by several LED lamps in the same color, such as three LED lamps in green. In case that the remaining battery capacity is larger than or equal to the first predefined capacity value Q1, all of the three lamps are on to indicate the full capacity; in the case that the remaining battery capacity is less than or equal to the second predefined capacity value Q2, only one is on to indicate the insufficient capacity; and in the case that the remaining battery capacity is less than the first predefined capacity value Q1 but is larger than the second predefined capacity value Q2, two are on to indicate the medium capacity.

Before the motor starts operating, the procedure of measuring and displaying the remaining battery capacity detects and judges the various parameters of the battery to determine whether the status of the battery can support the operation of the motor, and this could avoid the influence on the motor caused by the abnormal status of the battery and avoid the occurrence of an emergency. Calculating and displaying the remaining battery capacity after the motor starts operating, repeating the detecting and judging the various parameters of the battery, and calculating and displaying the remaining battery capacity monitors the operational status of the battery in real time and makes the discharge of the battery safer. Detecting and displaying the battery capacity can also be used to make the indication of the remaining battery capacity more accurate.

The remaining battery capacity displaying circuit of power tool, disclosed by the application, is not limited by the aforesaid contents described in the embodiments and structures shown in the drawings. Obvious alternations, replacements or modifications based on the invention to the components thereof fall in the protected scope of the invention. 

1. A battery capacity display assembly for a power tool, comprising: a first circuit module, arranged in the battery pack of a power tool for detecting and displaying a battery capacity; a second circuit module, arranged in the power tool for displaying the battery capacity; wherein, said battery capacity display assembly further comprises an optical signal transmitting device and an optical signal receiving device which communicate with said first circuit module and said second circuit module.
 2. A battery capacity display assembly for a power tool according to claim 1, wherein, said optical signal receiving device includes photo-electric elements.
 3. A battery capacity display assembly for a power tool according to claim 1, wherein, said optical signal transmitting device is an IR transmitting device.
 4. A battery capacity display assembly for a power tool according to claim 1, wherein, said optical signal transmitting device comprises a first light guide element having a first light guide surface, and said optical receiving device comprises a second light guide element having a second light guide surface, and when said battery pack is installed on said power tool, said first light guide surface is facing the said second light guide surface.
 5. A battery capacity display assembly for a power tool according to claim 1, wherein, said first circuit module further comprises a detecting circuit connected to said battery pack for detecting the battery parameters, a calculating control circuit for calculating, determining and controlling the electrical power supplied to the power tool according to the detected value of said detecting circuit, and a battery capacity indicating circuit which includes a battery capacity indicating device.
 6. A battery capacity display assembly for a power tool according to claim 1, wherein said second circuit module further comprises a battery capacity indicating circuit which includes a battery capacity indicating device.
 7. A battery capacity display assembly for a power tool according to claim 5, wherein said calculating control circuit comprises a CPU for performing calculations.
 8. A battery capacity display assembly for a power tool according to claim 5, wherein, said calculating control circuit comprises a MCU for performing calculations.
 9. A battery capacity display assembly for a power tool according to claim 5, wherein, said battery capacity indicating device is a multicolor LED lamp.
 10. A battery capacity display assembly for a power tool according to claim 6, wherein, said battery capacity indicating device is a multicolor LED lamp. 